The present invention relates to fuel cell devices.
More particularly, it relates to a fuel cell unit which has at least two fuel cell elements coupled with one another in series or in parallel, for converting chemical energy into electrical energy.
Fuel cell technology becomes even more important in connection with future vehicle concepts. Fuel cells provide the possibility of direct conversion of chemically bound energy directly into electrical energy, which subsequently can be transformed into a mechanical drive energy by means of an electric motor. In addition, the electrical energy of the fuel cell can be used in some cases for supply of various consumers both for mobile as well as for stationary applications.
In many cases hydrogen-enriched fuel can be generated for fuel cell unit from hydrocarbons, such as natural gas, gasoline, diesel or the like. For this purpose a corresponding conversion unit for converting hydrocarbon-containing material mixtures into a hydrogen-enriched fluid are utilized. Various methods can be used, for example the autothermic reforming, steam reforming, partial oxidation and the like.
A fuel cell unit is generally an electric and/or electronic circuit or coupling of several individual cells. In addition to the electrical circuit, a fuel cell unit includes also a structure which serves for the supply of the electrodes with starting materials and the transportation of the products. Fuel cell devices also include, in addition to the fuel cell unit, corresponding peripheral components, such as for example for gas supply and gas withdrawal, for heat management and for regulation or control.
In practice various identifications are used both for the fuel cell units as well as for individual fuel cells. In the subsequent description the term “fuel cell unit” will be used to include (total-) fuel cell stack or fuel cell pack, and the term “fuel cell element” will be used for individual (partial) fuel cells park or partial stack. What is important is that the fuel cell unit deals with a series and/or a parallel circuitry or coupling of individual fuel cell elements in the sense of the present invention.
When the fuel cell devices are used in vehicles, they deal with a great load spread from an idle running to a maximal load as well as numerous load interactions. In general the fuel cell devices, in particular the fuel cell units including the peripheral component, are designed for maximum required power. Fuel cell units in condition of relatively small loads have a higher efficiency than in condition of maximum loads. To the contrary, the total fuel cell device based on the periphery in the lowermost capacity region has a lower efficiency than in a small and average capacity region.
For example, when the drive motor does not work, for supplying the electrical system components with current or for vehicle air conditioning only relatively low powers, for example in the region of 0-5 kW must be made available from the fuel cell device. To the contrary, for providing the drive energy of the electric motor the power within the region of up to 70 kW and more is required.
In drive systems, the fuel cell devices in general operate with increased pressure of up to 3 bar, to increase the specific power of the system and to maintain the component dimensions as small as possible. The components which produce this pressure frequently have a very low efficiency for the mass current required in the partial load. This significantly increases the total system efficiency primarily in the power class which is relevant for the supply of the electrical system.
Moreover, the total system efficiency in the partial load region additionally reduces because of the cooling circuit of the system.
The relatively low current speeds in the partial load region present are a disadvantage since thereby the delay time during load changes is high and based on this the dynamic condition of the total system or the fuel cell device is significantly worsened.